Enzyme Regulation

Overview of Enzyme Regulation

  • Enzyme activity is controlled via two primary mechanisms:

    • Regulation of gene expression: Controlling the quantity of enzyme molecules produced.

    • Regulation of enzyme activity: Altering enzyme shape or blocking function with regulators.

  • Enzymes are protein catalysts that increase reaction rates by stabilizing the transition state and decreasing activation energy without altering equilibrium.

Enzyme Inhibitors

  • Competitive Inhibitors:

    • Bind to the active site, competing directly with the substrate.

    • Effect: KmK_m increases; VmaxV_{max} remains unchanged.

    • Can be reversed by increasing substrate concentration.

  • Non-competitive Inhibitors:

    • Bind to a site other than the active site (allosteric site), changing the enzyme's shape.

    • Effect: KmK_m remains unchanged; VmaxV_{max} decreases.

  • Irreversible Inhibition:

    • Inhibitors form covalent bonds with side chains in the active site, permanently inactivating the enzyme.

    • Examples:

      • Sarin: A nerve gas that forms a covalent bond with the R group of serine in the active site of acetylcholinesterase.

      • Aspirin: Binds to cyclooxygenase (COX), transferring an acetyl group to the active site to block Prostaglandin production.

Allosteric Regulation

  • Definition: Regulation where a non-substrate molecule binds to a site different from the active site, inducing a shape change.

  • Structure: Most allosteric enzymes have quaternary structure; the active site is on the catalytic subunit, while inhibitors/activators bind to regulatory subunits.

  • Cooperativity: A form of allosteric activation where the binding of one substrate molecule to one subunit locks all subunits into the active conformation.

Metabolic Pathway Regulation

  • Feedback Inhibition: A metabolic pathway is halted when its end product binds to and inhibits an enzyme that acts early in the pathway.

  • Key Example: Phosphofructokinase in glycolysis.

    • Stimulated by AMP (derived from ADP).

    • Inhibited by ATP and citrate.

  • Complex pathways (e.g., amino acid synthesis from aspartate) utilize multiple feedback points and distinct enzymes for the same initial reaction to avoid wasting intermediates.

Questions & Discussion

  • Question: Why does trypsin only hydrolyze peptide bonds next to lysine, and elastase only next to alanine?

  • Question: An enzyme from a mutant bacterium works at 20 ∘C20\,^\circ\text{C} but not at 37 ∘C37\,^\circ\text{C} (and remains inactive if cooled back down). What happened at the molecular level?

  • Question: Would you expect oxaloacetate to be a competitive or noncompetitive inhibitor of succinate dehydrogenase?

  • Question: What roles do allosteric regulation and feedback inhibition play in the metabolism of a cell?

  • Branched Pathway Logic: In a pathway where L forms M or N, M forms O, and O leads to P/Q or R/S:

    • If O inhibits the reaction of L to M, and products Q and S inhibit their respective branches from O, which reaction prevails if both Q and S are in high concentrations?

  • True or False: Allosteric enzymes have two or more binding sites. (True: they possess both catalytic active sites and regulatory allosteric sites).